By way of introduction, the principle of operation of a modern distance relay will first be described.
The distance relay is connected to transformers for measuring the current and voltage of the transmission line. On the basis of the amplitudes and phase positions of these quantities, the impedance of the line can be measured in the desired direction from the measuring transformer of the distance relay and be compared with a region of operation, set in that distance relay, in an impedance plane. The outer limit of the region of operation indicates the smallest impedance value which the transmission line can be expected to have during normal operation of the power supply system. When the line impedance lies within the region of operation a fault is present on the line and the distance relay is required to deliver a tripping pulse to the circuit-breakers.
The region of operation is often made in the form of a quadrangle in the impedance plane (the R-X-plane), and the limits of the region in the resistive and the reactive directions can normally be separately set.
The region of operation of the distance relay can also be extended in pre-set, time-dependent steps, whereby a backup protective function based on time selectivity can be obtained. In this way, a distance relay can comprise several protective or measuring zones. Broadly this functions in such a way that, for example, the distance relay operates instantaneously for faults within a line section closest to the measuring point. Within a second measuring zone, which comprises the line section of the first zone plus an additional line section, the distance relay operates after a certain, set time delay. Within a third measuring zone, comprising the line sections of the second measuring zone plus a further additional line section, operation is obtained in the event of a fault after an additional time delay, and so on.
The distance relays normally also have a direction sensing function. A transmission line which is fed from a plurality of stations can therefore be protected both against faults located ahead and against faults located behind (in relation to a station and a defined measuring direction). Therefore, distance relays located at respective ends of any section of the line also need to communicate with each other.
As mentioned above, distance relays are nowadays often constructed from static components operating digitally and controlled froma microprocessor. Even if these static components and the microprocessor have high reliability, faults or missing operations cannot, of course, be entirely avoided. Since failure to take protective action in the event of a line fault may have serious consequences, various ways of obtaining redundancy are attempted.
Redundancy is often obtained by the provision of parallel-operating protective relays having largely the same functions, possibly with different measuring principles, etc. Sometimes, parallel protective relays from different suppliers or relays having operating times of different duration are selected.
Another way of guarding against a distance relay failing to operate when required to do so, is to carry out test sequences of the tripping function. This can be done by means of external test equipment or by a self-monitoring or self-testing program built into the distance relay.
Testing of the operation of the distance relay can be performed at specified time intervals or according to other criteria, for example when the load states indicate stable conditions are existing.
Although the test times can be kept short and different more or less intelligent methods for determining safe times when the tests are to be performed have been developed, the fact remains that line faults occurring during the test time will not trigger a protective action. This entails an undesired uncertainty as regards the safe functioning of the distance relay, which may be felt to be undesirable.